Tire groove

ABSTRACT

A tire includes a tread portion having at least one circumferential main groove extending in a circumferential direction of the tire and spaced axially apart from an equator of the tire. The groove has a circumferential profile shape including an inner wall portion extending radially inward from an axially inner edge of the groove, an inner arc portion continuous with the inner wall portion and having a smaller radius of curvature than the inner wall portion, an outer wall portion extending radially inward from an axially outer edge of the groove and having a larger length than the inner wall portion, an outer arc portion continuous with the outer wall portion and having a smaller radius of curvature than the outer wall portion, and a groove bottom portion extending straight from the outer arc portion toward the inner arc portion while being inclined in a radially inward direction whereby the groove bottom portion has a constant radius of curvature.

FIELD OF INVENTION

The present invention relates to a tire and, more particularly, to atire including a tread groove with a cross sectional shape specified toreduce generation of cracks in the vicinity of the groove bottom.

BACKGROUND OF THE INVENTION

In a conventional tread portion of a pneumatic tire, a tensile straingenerates shrinkage of rubber in the groove bottom due to temperaturereduction after vulcanization. Such a strain may cause cracking at thegroove bottom. Particularly, cracking may occur at arc-like cornerportions between the groove bottom and groove walls, since the tensilestrain largely acts on the corner portions of the groove. Rubbershrinkage and cracking may also occur at arc-like corner portionslocated on a shoulder of the tread.

In order to conventionally suppress such cracking, a rubber with a highdegree of swelling in the tread portion has been used. However, rubberwith a high degree of swelling may degrade rolling resistance andsteering stability of the pneumatic tire, since rubber hardness maydecrease. Grooves with various cross sectional shapes have beenconventionally used to mitigate the cracking.

SUMMARY OF THE INVENTION

A tire in accordance with the present invention includes a tread portionhaving at least one circumferential main groove extending in acircumferential direction of the tire and spaced axially apart from anequator of the tire. The groove has a circumferential profile shapeincluding an inner wall portion extending radially inward from anaxially inner edge of the groove, an inner arc portion continuous withthe inner wall portion and having a smaller radius of curvature than theinner wall portion, an outer wall portion extending radially inward froman axially outer edge of the groove and having a larger length than theinner wall portion, an outer arc portion continuous with the outer wallportion and having a smaller radius of curvature than the outer wallportion, and a groove bottom portion extending straight from the outerarc portion toward the inner arc portion while being inclined in aradially inward direction whereby the groove bottom portion has aconstant radius of curvature.

According to another aspect of the tire, the groove has an axial treadsurface width between 10.0 mm and 20.0 mm.

According to still another aspect of the tire, the groove has an axialtread surface width of 14.84 mm.

According to yet another aspect of the tire, the groove has an upperbase radius between 3.0 mm and 8.0 mm.

According to still another aspect of the tire, the groove has an upperbase radius of 5.0 mm.

According to yet another aspect of the tire, the groove has a bottombase radii between 1.0 mm and 5.0 mm.

According to still another aspect of the tire, the groove has a bottombase radius of 3.0 mm.

According to yet another aspect of the tire, the groove has twoasymmetric draft angles of between 0.0° and 5.0° and 10.0° and 20.0°,respectively.

According to still another aspect of the tire, the groove has a draftangle radially inward of asymmetric draft angles between 10.0° and20.0°.

According to another aspect of the tire, the groove is located at anaxial position apart from the tire equator by a distance of 10.0 to 35.0percent of an axial tread width.

According to yet another aspect of the tire, the groove has a radius ofcurvature of the outer arc portion gradually decreasing away from aradially outer tread surface of the tire.

According to still another aspect of the tire, the groove extends at anangle of less than 45° with respect to the circumferential direction ofthe tire.

BRIEF DESCRIPTION OF THE DRAWINGS

The structure, operation, and advantages of the present invention willbecome more apparent upon contemplation of the following description ofexamples of the present invention taken in conjunction with theaccompanying drawings of the examples, wherein:

FIG. 1 is a cross sectional view showing a tread groove in accordancewith the present invention.

DETAILED DESCRIPTION OF EXAMPLES OF THE PRESENT INVENTION

A pneumatic tire may include a carcass extending from a tread portion toeach of two bead cores in opposing bead portions through sidewallportions, and a belt layer disposed radially outward of the carcass inthe tread portion, as disclosed in US 2012/0042998, incorporated hereinby reference in its entirety. The carcass may include at least onecarcass ply having a toroidal main portion that extending from one beadcore to the opposing bead core and turn up portions extending from bothends of the main portion and turned up around the bead cores from theaxially inside to the axially outside of the pneumatic tire to anchorthe carcass ply to the bead cores. In the carcass ply, carcass cords(e.g., carcass cords made of an organic fiber) may be disposed at anangle of, for example, 75° to 90° with respect to the tirecircumferential direction. Between the ply main portion and each turn upportion may be disposed a bead apex rubber for reinforcement of beadportions extending radially outwardly from the bead cores in a taperedmanner.

The belt layer may include at least two plies. The belt plies mayinclude belt cords having a high elasticity (e.g., steel cords). Thepneumatic tire may include two belt plies (e.g., a radially inner beltply and a radially outer belt ply in which the inner ply has a largerwidth than the outer ply) and steel belt cords arranged at an angle of,for instance, 15° to 40° with respect to the tire circumferentialdirection. The belt plies may be stacked so that the belt cords in oneply cross cords in the other belt ply.

The tread portion may have at least one circumferential main grooveextending continuously in the tire circumferential direction. The treadportion may have four circumferential main grooves extending straightand continuously in the tire circumferential direction on both sides ofa tire equator (e.g., a pair of first circumferential main groovesdisposed on a tread edge and a pair of second circumferential maingrooves disposed axially inward of the first circumferential maingrooves whereby on the tread portion are formed a pair of shoulder landportions extending between the first circumferential main groove and atread edge, a pair of middle land portions extending between the firstcircumferential main groove and the second circumferential main groove,and a center land portion extending between the second circumferentialmain grooves). The first and second circumferential main grooves may bedisposed symmetrically with respect to the tire equator.

The term “tread edge” as used herein means each of axially outer edgesof a ground contact surface of the pneumatic tire contacting a ground(flat surface) when the pneumatic tire, in a standard state, is loadedwith a normal load. The term “normal load” means a load defined forevery tire in a standardizing system and is, for example, a “maximumload capacity” in JATMA, a maximum value recited in the table of “TireLoad Limits at Various Cold Inflation Pressures” in TRA, and/or “LoadCapacity” in ETRTO, provided that in case of tires for passenger cars,the “normal load” is a load of 88% of the load defined above.

The first circumferential main groove may be formed into a straightgroove extending straight in the tire circumferential direction. Thestraight groove may define a shoulder land portion with a constantwidth. Therefore, since the tensile strain acting on the first maingroove may be equalized, cracking may be suppressed. However, the firstmain groove may not be limited to a straight groove, but may have otherconfigurations (e.g., a zigzag groove in which each zigzag component isinclined at an angle of 45° or 30° with respect to the tirecircumferential direction).

The first circumferential main groove may have a profile shape suchthat, in a meridian cross section including a rotational axis of thetire in a standard state, the profile shape may define an inner wallportion extending radially inward from an axially inner edge on one sideof the tire equator and defining a flat wall, or a curved wall having afirst radius of curvature, an inner arc portion continuous with theinner wall portion and having a smaller radius of curvature than asecond radius of curvature of the inner wall portion, an outer wallportion extending radially inward from an axially outer edge on a treadedge side and having a smaller length than that of the inner wallportion and defining a flat wall or a curved wall having a third radiusof curvature, an outer arc portion continuous with the outer wallportion and having a smaller fourth radius of curvature than the thirdradius of curvature of the outer wall portion, and a groove bottomportion extending straight from the outer arc portion toward the innerarc portion while being inclined in a radially inward direction wherebythe main groove has a maximum depth portion on a tire equator side withrespect to a center position of its width.

A pneumatic tire with such a circumferential main groove may increaserubber volume to enhance the rigidity near the outer arc portion. Thepneumatic tire may thereby counter tensile strain resulting from rubbershrinkage occurring after vulcanization and thus reduce cracks in thevicinity of the outer arc portion and the groove bottom. The pneumatictire may suppress increases in rolling resistance and deterioration ofsteering stability.

A “double bottom” groove geometry in accordance with the presentinvention may allow a larger base radius and/or larger draft anglewithout increasing the groove width at the tread surface. Limiting thegroove width may allow maintenance of wearable rubber volume (removalkilometers) and reduce rib tear from catching an obstruction with theinside surface of an outside rib. Both larger base radius and increasedraft angles result in lower stress at the bottom of the groove.

The double bottom groove geometry may allow the use of larger baseradius and and/or larger rib edge draft angle by eliminating therequirement that each side of the groove radii be tangent at the bottomof the groove. The upper and lower groove base radii may have a widerange of useable radii. Either may be varied independently to meetspecific application requirements and may also be impacted by designnonskid, rib draft angle, and appearance, while still meeting the sameobjectives of increasing the base radii and/or draft angle withoutincreasing the groove surface width. A lower limit for upper grooveradii may be a radii that is tangent at the groove base (full nonskid)with no sub groove being required. An upper limit may be restrained onlyby groove geometry and appearance. Lower groove radii may be restrainedby groove geometry and appearance. The radii and draft angles of FIG. 1set no constraints on possible combination options of radii and draftangles within the scope of the present invention.

One conventional double bottom groove has used a large base radius and anarrower, deeper subsection of groove bottom with a small base radius.This approach may increase under tread groove gauge (narrow subsurfacegroove prevents stones from reaching true groove bottom) and may allowlarger base radii and larger draft angles in the upper groove profile(reducing the probability of stone holding and reducing probability ofbase of groove stress cracking). However, the small base radius at thegroove bottom of the narrow subsurface groove affected nonskidmeasurement and visual appearance when approaching wear out.

A double bottom groove geometry in accordance with the present inventionmay minimize those disadvantages. Both the upper base radii and thebottom base radii may be large radii. As shown in FIG. 1, a doublebottom groove 10 of a pneumatic or non-pneumatic tire may have a treadsurface width 70 of between 10.0 mm and 20.0 mm, or about 15.0 mm or14.84 mm, an upper base radii 20 of between 3.0 mm and 8.0 mm, or about5.0 mm, and a bottom base radii 30 of between 1.0 mm and 5.0 mm, orabout 3.0 mm, along with asymmetric draft angles 40, 50 of between 0.0°and 5.0°, or 2.0°, and between 10.0° and 20.0°, or 15.0°, respectively,with a large draft angle 60 of between 10.0° and 20.0°, or about 15.0°,at a base of both sides of the groove 10.

While examples of the present invention have been described withreference to the drawing, the present invention is not limited to onlysuch examples and various changes and modifications may be made. Thepresent invention is more specifically described and explained by meansof the following examples and comparative examples. It is to beunderstood that the present invention is not limited to these examples.

What is claimed:
 1. A tire comprising a tread portion having at leastone circumferential main groove extending in a circumferential directionof the tire and spaced axially apart from an equator of the tire, thegroove having a circumferential profile shape including an inner wallportion extending radially inward from an axially inner edge of thegroove, an inner arc portion continuous with the inner wall portion andhaving a smaller radius of curvature than the inner wall portion, anouter wall portion extending radially inward from an axially outer edgeof the groove and having a larger length than the inner wall portion, anouter arc portion continuous with the outer wall portion and having asmaller radius of curvature than the outer wall portion, and a groovebottom portion extending straight from the outer arc portion toward theinner arc portion while being inclined in a radially inward directionwhereby the groove bottom portion has a constant radius of curvature. 2.The tire as set forth in claim 1 wherein the groove has an axial treadsurface width between 10.0 mm and 20.0 mm.
 3. The tire as set forth inclaim 1 wherein the groove has an axial tread surface width of 14.84 mm.4. The tire as set forth in claim 1 wherein the groove has an upper baseradius between 3.0 mm and 8.0 mm.
 5. The tire as set forth in claim 1wherein the groove has an upper base radius of 5.0 mm.
 6. The tire asset forth in claim 1 wherein the groove has a bottom base radii between1.0 mm and 5.0 mm.
 7. The tire as set forth in claim 1 wherein thegroove has a bottom base radius of 3.0 mm.
 8. The tire as set forth inclaim 1 wherein the groove has two asymmetric draft angles of between0.0° and 5.0° and 10.0° and 20.0°, respectively.
 9. The tire as setforth in claim 1 wherein the groove has a draft angle radially inward ofasymmetric draft angles between 10.0° and 20.0°.
 10. The tire as setforth in claim 1 wherein the groove is located at an axial positionapart from the tire equator by a distance of 10.0 to 35.0 percent of anaxial tread width.
 11. The tire as set forth in claim 1 wherein thegroove has a radius of curvature of the outer arc portion graduallydecreasing away from a radially outer tread surface of the tire.
 12. Thetire as set forth in claim 1 wherein the groove extends at an angle ofless than 45° with respect to the circumferential direction of the tire.